Gas utilities that operate older cast iron systems often find it necessary to perform repair or replacement maintenance on these systems. In the case of low pressure gas mains, which typically operate at approximately 7 inches (in.) of water column—i.e., at about ¼ psig—the work may be performed “live”. In such a case, gas flow may be blocked off on both sides of a repair site, while leaving gas supply to other portions of the system in the surrounding area unimpeded.
In order for customers to be unaffected while the work is performed, it is necessary to have adequate gas supply on both sides of the blocked-off work area. To ensure that adequate gas supply is available, a test, sometimes referred to as a flow test, may be performed on both sides of the blocked-off work area before the work begins. A flow test may include installing a bypass around the affected area of the pipeline and then stopping gas flow through the pipeline, thereby forcing the gas to flow through the bypass. A “bypass” is a temporary section of pipe that allows the gas to circumvent the main section of piping during maintenance activities. Gas flow through the pipeline may be stopped, for example, by using a bag stopper. Pressure gauges—e.g., manometers—are installed in the pipeline on either side of the stopper, for example, in the bypass piping, so that the pressure on each side of the affected area can be measured before work begins.
When all of this is installed, a valve in the bypass is shut so that gas flow is effectively stopped on each side of the affected area of the pipeline. At this point, the manometers are measuring the pressure of the gas on each side of the affected area; however, this may not be adequate to determine if customers may be adversely affected by the shutdown. This is because gas usage could increase while the work is being performed, and the gas pressure that was previously considered adequate could drop to an unacceptably low level on one side of the affected area. Therefore, in order to simulate a spike in gas usage, some of the gas from the pipeline on one side of the stopper is vented to the atmosphere generally using a valve connected to the bypass that is open on one side to atmosphere. This venting may continue for 2-3 minutes or more to simulate a realistic gas loading condition. If, during the venting, the measured pressure on the vented side remains above a predetermined level, the gas flow for that side of the affected pipeline area is considered adequate.
Following the venting process, or during an interval during the venting, the venting may be stopped by closing the valve, and manometers may be monitored. To ensure that the side of the main under test has adequate flow to support increased demand, the manometer should, upon closing the valve, quickly return to the nominal operating pressure of the main for example, 7 in. of water column. If the pressure does not return to the nominal level, it may be assumed that there is not sufficient flow from one side of the main to support the increase in demand. This would indicate to the utility that the bypass must stay in place during the repair/maintenance activity. This venting and measurement procedure is repeated on the other side of the stopper to ensure the pressure on that side is also adequate.
Although this method has been shown to be effective, it would be desirable to have an alternative that does not require venting large amounts of natural gas into the atmosphere. Such a system could provide, for example, reduced repair costs, environmental benefits, and a better public perception for the utility. Therefore, a need exists for an apparatus and method that facilitate pressure testing under simulated load conditions similar to those described above, but without requiring large amounts of gas to be released into the atmosphere.